Currently, most of the modern communication systems are based on the fiber optical communication network, and fiber optical network has offered unprecedented huge capacity and installation flexibility and is able to support a variety of broadband applications that are under ceaseless development. Broadband and multi-channel tunable laser could help maximize the present fiber optical network resources. Data traffic can be transferred from a congested channel to an unused channel by means of dynamic provision of broadband, thus Internet requirements are met. Use of a tunable laser makes rapid establishment or change of a light path easier, and it has become one of the important sources for implementing a dynamic fiber network.
In view of these applications, an ideal tunable laser shall include the characteristics described below: wide tunable range, i.e. covering waveband C and(or) waveband L (approximately 1520 nanometers to 1620 nanometers); small size; fast handoff (sub-millisecond level) between the frequency intervals of any two international Telecommunication Unions (ITU); excellent long-term working stability (service time over 25 years); high reliability under extreme environmental conditions; low power consumption; and easy manufacturing and low cost.
With the successful development of Dense frequency Division Multiplexers (DWDM) and other high density spectrum-related sources, the modern optical system has been developed to a system with a frequency interval of 100 GHz, 50 GHz, 25 GHz and even higher density from the previous system with a frequency interval of 400 GHz and 200 GHz; meanwhile, optical communication system's transmission rate has also been developed to 10 Gbps, 40 Gbps or 100 Gbps from 2.5 Gbps in the past. This results in corresponding requirements on small-size tunable laser for the optical communication system, and in particular, higher requirement on spectrum density and spectrum width of laser output light. An external cavity tunable laser with high-sharpness etalons can achieve the aforementioned requirements and therefore suitable for the new generation dynamic optical communication system with high transmission rate and high spectrum density.
In the external cavity tunable laser, especially in such sources for optical communication, in addition to use of the high-sharpness etalons, a matched tunable narrow band optical filter is also required for compression of laser output bandwidth. If the frequency interval of laser output light is required to be Δƒ, the filtering bandwidth of the optical filter shall not exceed twice of this frequency interval, i.e. <2Δƒ, thus avoiding the laser working under a multi-mode state and further improving the operating stability of the laser; if the frequency interval of laser output light is required to be 50 GHz, the filtering bandwidth of the optical filter should be less than 100 GHz; if the frequency interval of laser output light is further required to be 25 GHz, the filtering bandwidth of the optical filter should be less than 50 GHz. An optical filter with narrower bandwidth means higher manufacturing difficulty and higher cost, such as conventional optical grating filter and acousto-optic filter. Similarly, an etalon with narrower transmission spectrum interval means larger size, higher manufacturing difficulty and higher cost.